Hi, We just did out ILR, I was surprised that we can submit scans, they check though the documents at your appointment you book when you submit the visa application. We had some trouble at first getting the file size down but were ok after we found their recommended settings.
They recommend scanning at 150dpi-300dpi in greyscale (we did all ours at 150dpi). They ask for pdfs but pngs and jpgs are ok to. We put our multipage documents together as a pdf, passports etc and any single page documents as jpgs. Each file has to be under 6mb.
Good luck with your application